Complex permittivity is the constitutive property required to fully define the absorption, reflection, and transmission of microwave frequency electromagnetic energy for a non-magnetic material. We report the complex permittivity of 1,3,5-triamino2,4,6-trinitrobenzene (TATB) powder from 1 to 18 GHz. The average complex permittivity of individual TATB particles was estimated from measurements of the complex permittivity of the powder at two different densities. TATB was found to have low permittivity, low dielectric loss with nearly constant valued permittivity between I and 18 GHz. These data are used to calculate the complex permittivity of a composite composed of TATB and Kel-F 800.